1. Field of the Invention
The present invention relates to a radiographic image detector that detects an image of a subject from radioactive rays penetrating through the subject.
2. Description of the Related Art
In the medical field, radiography using radioactive rays like x-rays has been well known as a diagnostic method. An x-ray radiography system generally consists of an x-ray source for radiating x-rays and an x-ray image detector for detecting an x-ray image of a subject from x-rays penetrating through the subject. Among those used in practice, there is an x-ray image detector that adopts a flat panel detector (FPD). The FPD includes a TFT (thin film transistor) active matrix substrate, in which pixels (sensor elements) having a thin film transistor each are arranged in an array for accumulating signal charges in respective pixels corresponding to the amounts of x-rays incident thereon. The FPD outputs the accumulated signal charges in the form of digital image data representative of the x-ray image of the subject.
Unlike conventional x-ray image detectors using radiographic film or an imaging plate, the FPD type x-ray image detector needs to be controlled so as to synchronize its charge accumulating operation with the timing of x-ray radiation from the x-ray source. For this purpose, a control device for the x-ray image detector, like a console, is configured to receive a start-of-radiation signal from a radiation starting switch when the radiation starting switch is operated to actuate the x-ray source to start x-ray radiation. The control device transfers the start-of-radiation signal as a synchronizing signal to the x-ray image detector, upon which the x-ray image detector starts accumulating signal charges for the imaging.
However, when a radiography system adopts such an x-ray image detector that is produced by a different manufacturer from that of an x-ray source, the interface of the image detector may sometimes be incompatible to the interface standards of a control device for the x-ray source (standards of cables and connectors, format of the synchronizing signal, etc.). To solve this problem, various automatic exposure control (AEC) methods have been disposed, whereby the start of x-ray radiation from the x-ray source may be detected on the side of the x-ray image detector so that the image detector may be synchronized with the x-ray source without the need for the synchronizing signal from the x-ray source.
For example, JPA 2005-143802 discloses utilizing parts of pixels of the x-ray image detector as detective pixels for detecting the end of x-ray radiation. Specifically, output values from these partial pixels are detected in the beginning of x-ray radiation, and from among these pixels, one outputting the highest value, representative of the largest amount of radiation, is selected as a reference pixel for judgment. Then the output of this reference pixel is monitored at regular intervals, to detect the end of x-ray radiation when the output level of the reference pixel stops increasing.
According to the above prior art method, however, since a large number of detective pixels are distributed over the whole imaging area of the image detector, it takes a certain time for selecting the reference pixel from among all of these detective pixels.
In addition, as a matter of fact, there may be variations in sensitivity between the detective pixels. If the sensitivity of the selected reference pixel is low, the end of radiation cannot be detected with sufficient accuracy. This may cause a problem especially when the radiography should be done at a reduced dosage level.
The same problem can occur if the start of radiation would be detected using such a reference pixel as above, although the mentioned prior art does not serve for detecting the start of radiation.